1. Field of the Invention
The present invention is a process for metering permeated hydrogen flow in machines, equipment, piping, or other metallic apparatus, used in the oil industry, refineries, chemical industries, petrochemical industries, units for production, pumping, transport, and storage of petroleum and gas, tanks, machines, and equipment that work with hydrogen, or chemicals that can generate hydrogen, and nuclear industries. The system uses a sensor that uses the properties of a couple of dissimilar materials that are, in construction and installation, suitable to measure electrical values between a metering couple and a reference couple. The measured value is a function of the flow rate of hydrogen that permeates the metallic surface being monitored. In consequence, we obtain a process for measuring hydrogen corrosion or hydrogen flow through an apparatus that has a low cost of construction, an unlimited shelf life, does not waste any materials, has a high response velocity, and has an accuracy and precision that are equivalent to or better than those obtained by the state-of-art methods. The system is extremely simple and cheap to install, and has a low cost of maintenance, and is easily integrated with process computers.
2. Description of the Prior Art
As is well known in the field of Industrial Corrosion sector, structural damage is caused, in many cases, by the intrusion of hydrogen in a metallic structure. The hydrogen is generated by acidic means containing free protons (H+cation), through chemical processes that lead to proton formation, by atomic hydrogen (H0) formation, or even by hydrogen gas (H2) being adsorbed in the metallic structure. The structural damage caused by the hydrogen is quite varied, and includes hydrogen-induced cracking (HIC), sulfur stress cracking (SSC), and Stress Oriented Hydrogen Induced Cracking (SOHIC). Several processes have been conceived for controlling these problems, such as forming a layer of protective material, controlling the conditions of the fluid in contact with the material submitted to corrosion, etc. However, a key problem is how to measure, in a safe, economically viable, and quick way, the hydrogen formation next to a corrosion-subjected surface. Great efforts have been made to obtain a hydrogen sensor with a faster response time, with easy installation, the least possible maintenance, with precise and accurate results, with integration to data processing systems, and, of course, the least possible cost.
Current arat sensors for hydrogen permeated in metallic structures can be classified into 4 groups: Pressure sensors, vacuum sensors, electrochemical sensors, and fuel cell sensors as described below:
1. Pressure sensors are based on measuring the pressure generated by gaseous hydrogen (H2), formed by the combination between hydrogen atoms (H0), when these atoms cross the hydrogen-permeated surface, or the walls of a reaction tube inserted in the hydrogen generating means. These sensors can be of 2 types:
1.1 Pressure sensors by insertion: This model is made of a thin-walled carbon steel pipe (reactional tube), which has one of its ends closed, while the other end is in communication with a pressure meter, the meter being inserted into the hydrogen generating means. In FIG. 4, we can see a typical pressure sensor by insertion, which has a pressure meter (11), typically a manometer, a connection (12), an external body (13 and 14), a reaction tube (15), inserted in the hydrogen generating means (16). Atomic hydrogen (H0), formed by corrosion reactions out of the wall of reactional tube (15), cross this wall and then changes to molecular hydrogen gas (H2), with a molecular volume greater than H0. The gas can therefore not return to the hydrogen generator means, and thus accumulates inside the tube, raising the tube internal pressure, which is measured by the pressure meter (11). That sensor allows checking the efficiency of corrosion inhibitors based on the suppression of hydrogen formation, having pressure stabilization when an inhibitor is effective. However, these sensors do not have a quick response time (it can even take one month to attain measurable levels), nor great sensitivity. In addition, these sensors are difficult to integrate with process computers.
1.2 External pressure sensors: This model of sensor works similarly to those described above, but the sensor is installed externally, forming a chamber between the external wall of the corrosion-subjected surface and the sensor, where molecular hydrogen (H2) accumulates, giving rise to the pressure increase, in the same way as in insertion-type sensors. In FIG. 5, a typical external pressure sensor is shown, with an external coupling (21), a manometer-thermometer assembly (22), a pressurizing chamber (23), with this assembly being coupled directly to the surface under corrosion by hydrogen (24). This sensor has the advantage, over the insertion-type sensor described in item 1.1, that it can be assembled externally to the corrosion-subjected surface, without interference on the industrial process. However, the system response time is even slower, due to the greater thickness of the corrosion-subjected walls, when compared with the wall thickness of the reaction tube from the insertion-type sensors. The system also suffers the other shortcomings of the insertion-type sensors.
2. Vacuum-type sensors: These sensors are based on the changing in the grid current of a vacuum electronic valve, when its exterior side, made in steel, suffers corrosion by hydrogen, this current being proportional to the mass of hydrogen coming into the tube. They can be installed externally to the surface under corrosion as well as through insertion in the corrosive means, and have been greatly improved ultimately, having, over the pressure sensors, the advantage of a greater sensitivity. State-of-the-art Vacuum sensors work with the hydrogen-collecting cavity under high vacuum (10xe2x88x926 Pa), and they can measure hydrogen masses as low as 10xe2x88x929 g. In spite of its greater sensitivity, however, vacuum sensors are indicated only for laboratory work, or in industrial units with a very controlled environment, like, for example, in nuclear plants, due to its electronics and hardware being very expensive and fragile for the rough working conditions of an oil plant.
3. Electrochemical sensors: Beginning from the work of Devanathan et alii, which aimed at first to determine diffusivity of hydrogen through metallic plates, using an electrochemical double cell, in which the metal test-piece was the surface separating the semi-cells each other, electrochemical sensors were developed based on the oxidation of atomic hydrogen (H0) and electrochemical reduction of the formed ionic hydrogen (H+) producing molecular hydrogen (H2), the electrical current from that oxidation being proportional to the mass of permeated hydrogen. A commercial example of this type of sensor is shown in FIG. 6. In this model, atomic hydrogen (31) that permeates the corrosion-subjected surface (32) is oxidized when permeating a palladium metal sheet (33), polarized by a potentiostate, forming hydrogen cathion (H+) when entering into contact with the electrolyte (34). The hydrogen cation produced is then reduced in the auxiliary electrode (35), forming molecular hydrogen. In this model, the main disadvantage is the use of a noble metal (palladium), with the necessary cost increase. Electrochemical sensors are generally of complex construction, need expensive measurement instruments, have a low response velocity, need an external assembly, and have the additional disadvantage of a limited shelf life. In addition, the electrochemical processes can be very complicated, being subject to interference by generator means and electrolytes contaminants, by the temperature, etc. Several variants have been recently developed, such as solid electrolyte sensors, but none of these efforts actually eliminated the cited disadvantages.
4. Fuel cell sensors: This type of sensor makes use of the fuel cell principle, where there is electrical current generation when the hydrogen generated by the corrosive means crosses over the surface under corrosion (anode) in the atomic form (H0), and is transformed in ionic hydrogen (H+) by entering in contact with an electrolyte, and then reacts with oxygen from the air in a porous cathode, forming water and thus generating the electrical current. Once each hydrogen atom provides one electron, that current is proportional to the flow of hydrogen by the surface. An example of this type of sensor, utilizing as the solid electrolyte a proton-exchange membrane of perfluorinated sulphonic acid, is seen in FIG. 7, where the corrosion-subjected surface whose hydrogen flow is to be measured (41) corresponds to the fuel cell anode, the point of admittance of hydrogen (42), the membrane-type solid electrolyte (43), the porous electrolyte (44), that catches oxygen from the air and corresponds to the cathode from the fuel cell, and the current collector (45), which is electrically connected, as well as the material under corrosion (41), to a microamperimeter for measuring the electrical current proportional to hydrogen flow. In order to obtain the greatest possible transport of oxygen from the air, the cathode is made from graphite pressed with platinum particles with a large contact surface, making the sensor expensive. Besides that, the mechanical construction is relatively complex, raising the cost for manufacturing of this model. Finally, this type of sensor does not actually eliminate the disadvantages of the electrochemical sensors, as it requires an external assemblage, with consequent delay in response time, and complex and expensive measuring instruments.
The present invention was developed to overcome the disadvantages of the sensors and processes now in use by utilizing the concepts of basic instrumentation, such as thermocouples, for a novel application. The system uses the physical properties of the coupling of dissimilar materials, and utilizes two parts, one of the parts being the metering couple. The metering couple is welded on the metallic surface in contact with the hydrogen-generating means which one wants to measure, in such a way as to form a metallurgical continuity with that surface, or bonded to the surface in any other way which allows diffusion of hydrogen through the couple, so as to be subjected to permeation by hydrogen. The other part of the system is the reference couple, attached to the face of the metallic surface in contact with the hydrogen-generating means, so as to form no metallurgical continuity with that surface, so that no hydrogen passes through the surface. The metering couple and the reference couple are both connected to meters of electrical units, such as electrical potential, in that the difference of the electrical units between the couples is a function of the hydrogen mass flow through the surface. In consequence, a process for measuring hydrogen mass flow through a sensor that is easy and cheap to construct and install is obtained. The system can be assembled externally (in such case the metallic surface under permeation by hydrogen being the monitored item""s own surface) as well as by insertion (in such case the metallic surface under hydrogen permeation being a thin-wall reaction tube inserted in the process fluid, taking advantage of the lower response time relative to the external assemblage). The system has a very low cost of maintenance, with an unlimited shelf life, and obtains a high response velocity, an accuracy and precision that are equivalent or better than those obtained by the state-of-art methods, and a extremely simple and cheap installation, a low cost of maintenance, with easy integration with process computers, either digital or analog.